1. Field of the Invention
The present invention generally relates to improvements in light valve projection systems of the Schlieren dark field type and, more particularly, to an improved electron gun for the light valve to achieve improvements in the depth of focused field of the electron beam and a reduction in beam spread and off-axis aberrations of the beam, as well as less criticality in mechanical alignment of electro-optical parts and applied voltages.
2. Description of the Prior Art
Light valve projection systems of the Schlieren dark field type have been in commercial use for many years and are capable of providing excellent performance. Typical prior art color projection systems of this type are shown in U.S. Pat. Nos. 3,290,436, 3,352,592 and 3,437,746, all of which were issued to W. E. Good et al. The principles of operation of this type of projection system are briefly described with reference to FIGS. 1, 2 and 3 of the drawings.
With reference first to FIG. 1, there is schemetically shown a single-gun television light valve assembly comprising a lamp 10, sealed light valve 12, and Schlieren projection lens 14. The sealed light valve 12 comprises a glass envelope which contains an electron gun 16, input slots 18, focus-deflection system 20, a control layer 32 on a rotating disk 22, and a fluid reservoir 24. A specific example of an electron gun used in light valves of the Schlieren dark field type is disclosed in U.S. Pat. No. 3,586,901 to Findeisen.
The electron gun 16 generates, from anode aperture 11, an electron beam which is used to "write" charge patterns on the control layer 32. It should be understood that disk 22 is made of glass and, on the side facing electron gun 16, has a transparent electrode surface which is electrically connected to a source of positive potential with respect to the cathode of the light valve. Disk 22, and its transparent electrode, are coated with a layer of deformable fluid which is the control layer 32. Electron charge patterns from the electron beam are deposited on the surface of the control layer 32 and are acted upon by the electric field from the disk electrode to deform the surface of the control layer, forming diffraction gratings. The electron beam is focused, deflected and modulated to control the fluid surface deformations which control the light rays passing through the layer 32 and disk 22.
The focus-deflection system 20 comprises three electrode sets each having four orthogonal electrodes, which form three electrode "boxes", referred to as boxes 23, 25 and 27, and a cylindrical electrode 21. The first of these, box 23, is arranged about the aperture in the input window and serves to center and allow pre-deflection of the electron beam. The next two boxes, boxes 25 and 27, have DC and AC voltages applied to them in a manner to achieve a uniformly focused electron beam image of aperture 11 which is scanned across the raster plane on control layer 32. This, in turn, permits the control layer fluid to be modulated uniformly by charge control to produce a uniformly colored projected image. following the focus-deflection boxes 25 and 27 is a drift ring 21 which serves, with a transparent electrode on disk 22, as an element of the final electron lens in the focus-deflection system 20.
Specific examples of light modulating fluids are disclosed in U.S. Pat. No. 3,288,927 to Ralph W. Plump, U.S. Pat. Nos. 3,317,664 and 3,317,665 both to Edward F. Perlowski, Jr., U.S. Pat. No. 3,541,992 to Carlyle S. Herrick et al, and U.S. Pat. No. 3,761,616 issued to C. E. Timberlake. These fluids may include additives as taught by U.S. Pat. Nos. 3,764,549 and 3,928,394 to David A. Orser. In general, the control layer or light modulating fluid is a very special chemical compound, modified with special additives, having the electro-mechanical and visco-elastic properties needed to produce effective control layer properties in the electron beam addressed light valve.
The basic light collection system includes an arc lamp 10, which may be a Xenon lamp, the arc of which is located at the focus of a reflector system, which may be a simple ellipsoidal reflector, as shown, or a compound reflector, as disclosed for example in U.S. Pat. No. 4,305,099 to Thomas T. True et al. The light from the arc is reflected from the reflector through a pair of spaced lens plates having corresponding pluralities of rectangular lenticules arranged in horizontal rows and vertical columns. The first lens plate is shown in FIG. 1 at 28 and the second lens plate is formed on the light input surface of the glass envelope of the light valve 12. The light from the lamp 10 is projected through a color filter plate 26 and the lenticular lens 28 before entering the light valve 12.
The interior surface of the glass envelope of the light valve 12 carries the input light mask in the form of slots 18 which, for example, may be applied by vapor deposition. The input slots 18 are a series of transparent slots and alternating opaque bars in a pattern generally as indicated in FIG. 1. The filtered light rays from the lamp 10 pass into the light valve 12 through these transparent slots. The lenslets of the lenticular lens 28 and the corresponding lenslets, formed on the light input surface of the glass envelope of the light valve 12, form condensing lens pairs which first focus spots of filtered light onto the slots of the light mask and then re-image the light rays onto the control layer raster plane 32. With this arrangement, efficient utilization is made of light from the arc lamp, and uniform distribution of light is produced, in a rectangular pattern, on the light modulating medium or control layer 32.
The Schlieren projection lens 14 includes Schlieren lens elements 29, output color selection bars 30 and a projection lens system 31. The output selection bars 30 are the complement of the input slots 18. That is, on the output bar plate, the bars are optically aligned with the slots of the input slots 18 so that, in the absence of a diffraction of light passing through the control layer 32, light rays are focused and terminated on the bars of the output bar plate. This creates a "dark field" condition, i.e., no light is transmitted in the absence of a modulating signal superimposed on the raster scanning signals applied to the horizontal and vertical deflection plates of the deflection system 20. It should be noted, however, that the electron beam which scans the raster and provides charge to the control layer is a constant current electron beam, there being no modulation of the intensity of the beam produced by the electron gun 16 (other than during the horizontal and vertical retrace intervals when the beam is off).
The lower half of FIG. 1 shows the cross sections of the light body and light valve components. The spectral diagrams at the bottom indicate how the light is prefiltered before entering the light valve.
FIG. 2 is a simplified light valve diagram showing the color selection action of the three basic gratings. The control layer 32 which is supported by the rotating disk 22 (shown in FIG. 1) is illustrated as having three different diffraction gratings for red, green and blue light components. These diffraction gratings may be written individually or simultaneously and normally are actually superimposed but, for purposes of illustration only, they are shown in FIG. 2 as separated on the control layer 32.
In the light valve projection system shown in FIGS. 1 and 2, green light is passed through the horizontal slots of the input bar plate 18 and is controlled by diffraction gratings formed by modulating the height of the scanned raster lines on the control layer 32. This is done by controlling the amplitude of a high frequency carrier applied to the vertical deflection plates as modulated by the green video signal as shown in FIG. 3. Magenta (red and blue) light is passed through the vertical slots of the input bar plate 18 and is controlled by charge generated diffraction gratings created at right angles to the raster lines by velocity modulating the electron spot as it is scanned in the horizontal direction. In the example shown in FIG. 3, this is done by applying a 16 MHz (12 MHz for blue) signal to the horizontal deflection plates and modulating it with the red video signal as shown in FIG. 3. The grooves created in the control layer 32 have the proper spacing to diffract the red portion of the spectrum through the vertical output slots in plate 30 while the blue portion is blocked. (When the 12 MHz carrier is used, the blue light is passed by the vertical slots in plate 30 and the red light is blocked.)
Thus, three simultaneous and superimposed primary color pictures can be written with the same electron beam and projected to the screen 33 as a completely registered full color picture. Colors are created by writing miniature diffraction gratings within each picture element on the fluid surface by manipulating the single scanning electron beam. These gratings diffract the transmitted light rays away from their terminations at the output bars where they are spatially filtered to let the desired color reach the screen. The amount of light diffracted is dependent on the depth of the gratings formed in the control layer. This technique permits a full color television picture to be written on a single control layer with no need for further registration.
FIG. 3 shows in block diagram form the basic light valve projector circuitry. A composite video signal is supplied to the input of a decoder 34 which provides at its output red, blue and green video signals. These signals are respectively applied to modulators 36, 38 and 40. A grating generator 42 supplies carrier signals which, in the case illustrated, have frequencies of 16 MHz and 12 MHz, respectively, to modulators 36 and 38 and a signal having a frequency of 48 MHz to modulator 40. The outputs of the red and blue modulators 36 and 38 are combined and superimposed on the horizontal deflection signal from the horizontal deflection signal generator 44. The output of the green modulator 40 is superimposed on the vertical deflection signal from the vertical deflection generator 46.
The basic Schlieren dark field light valve projector as schematically illustrated in FIGS. 1, 2 and 3 has evolved over a period of years to be a highly efficient projector producing excellent quality pictures of good color balance and high resolution. There is, however, an ongoing effort to improve and optimize the design and operation of the projector. Among the more critical design considerations is the electron optics of the light valve. The electron optics are, as may be appreciated from the foregoing discussion, quite complex and dynamically changing as a result of the varying deflection voltages.
It has been found that with very exact alignment of the light valve's focusing and deflecting electrode system and with critically close control of applied voltages, sweep balance and dynamic pre-deflection, reasonably acceptable video writing performance can be obtained. Even with the aforementioned concerns under careful control, prior art electron optics have required operational compromises to be made in balancing the many variables of electrical and mechanical properties of the light valve system for best performance. Accordingly, there is a continuing need to optimize and improve the uniformity of the modulated color fields of the projected images and to decrease the sensitivity of the mechanical assembly and alignment of the electron optics electrode system.
The present invention approaches optimization of the design of the light valve by making improvements in the electron gun. Electron guns known in the prior art include those disclosed in the following U.S. patents.
U.S. Pat. No. 2,888,605 to Brewer discloses a system which forms a "collimated flow of electrons" that converges electrons through an accelerating electrode 32 and then through a drift ring cylindrical electrode 20 to reduce transverse velocity components and increase the lateral velocity. A range of voltages of 1.1 to 1.5 is identified. No beam shaping aperture is used. The cylindrical electrodes fields create a drift space but do not intercept portions of the cathode emitted beam.
U.S. Pat. No. 2,909,704 to Peter discloses a system which produces a divergent beam from the cathode. This is achieved with a first positive aperture anode, B, and a negative focusing ring, A. voltages applied to these two electrodes cause the divergent beam to be focused into a "parallel" flow. Subsequent anodes have aperture diameters substantially greater than the diameter of the cathode electrode and the voltages of the first two accelerating electrodes are of the same order. Additionally, the focusing electrode surface is behind the plane of the cathode. The objective of Peter's gun design is to produce a "low noise" electron beam by minimizing axial velocity spread. This is accomplished with the use of large anode apertures which do not intercept significant portions of the electron beam. Peter generates a beam which is from one to four times the diameter of his cathode.
U.S. Pat. No. 3,349,269 to Hamann discloses a system that employs converging beam crossovers in a cathode ray tube that is used to generate alphanumeric characters. The electron beam is used to flood a beam shaping matrix which forms the alphanumeric characters, and the resulting beam is deflected by two pairs of cylindrical lenses to the desired target position. The electron gun does not produce a laminar flow with minimum beam angle.
U.S. Pat. No. 3,417,194 to Yoshida et al discloses a cathode ray tube which seeks to reduce beam spread by compensating for the defocusing effect of relatively high video modulation voltages which are applied to the electron gun grid-cathode region. There is no generation of laminar or parallel flow of the electron beam. Grid electrode apertures are of the same or larger diameter as distance from the cathode is increased.
U.S. Pat. No. 3,586,901 to Findeisen discloses an electron gun of the general type which has been used in light valves of the type described. This gun is a beam cross-over type using a beam shaping anode aperture to shape the emitted electron beam. It further uses a small "potential hill" field, downstream from the cathode, grid and apertured anode, to reduce the number of positive ions which can reach the aperture and emitter. These positive ions result from the electron beam bombarding the control layer on the rotating disk of the light valve.
U.S. Pat. No. 3,740,607 to Silzars discloses a laminar flow electron gun which provides a small beam with high current density. This is accomplished by using long focal length lenses without limiting apertures and seeks to establish the screen spot size by imaging the virtual cathode of the gun with converging lenses. The aperture used in the gun anodes are substantially larger than the diameter of the beam. There are no beam intercepting apertures in Silzers' gun nor is the beam shaped by current limiting apertures.
U.S. Pat. No. 3,924,153 to McIntyre discloses an electron gun which seeks to prevent beam spread by using an axial cylindrical electrode surrounding the grid to anode region of the gun to reduce radial velocity components of the beam. While McIntyre does use a beam limiting aperture, there is nothing about the design that would produce a laminar flow and minimize the beam spread angle nor is there any use of multiple apertured anodes which shape the beam's cross-section. Further, the aperture used is not imaged to the target.
U.S. Pat. No. 3,980,919 to Bates et al discloses the formation of a sheet beam. The effects of asymmetries are minimized by avoiding small apertures and using long focal length lenses through the central portion of the lenses in the gun. Apertured electrodes are used to create needed accelerating fields, but they do not intercept or limit the beam. None of the apertures are imaged to the target.
U.S. Pat. No. 4,467,243 to Fukushima et al discloses an electron gun which uses an apertured grid and anode in which the diameter of the first aperture is smaller than or equal to the diameter of the second aperture. By this means, there is formed a uniform axial field and a laminar flow electron beam of constant current density is generated. The spot size and shape is produced by magnetic focus, not by an aperture.
U.S. Pat. No. 4,481,445 to Gorski teaches a bipotential electron gun which generates a cross-over beam establishing fields through a series of apertured electrodes of increasing aperture diameter from the cathode.
U.S. Pat. No. 4,496,877 to Kueny teaches a gun construction which tries to counter the beam spread angle effects of a first cross-over with field shaping electrodes and voltages. Apertures are used for beam focusing field formation and not beam interception. Aperture diameters are the same or larger as distance from the cathode increases.